The present invention relates to a process and apparatus for the gas phase exothermic reaction of a feed gas mixture. More particularly, the invention is directed to a process and apparatus for the catalytic combustion of combustible gases.
Natural gas and other combustible gases are generally combusted in flame at temperatures of 1300-1600.degree. C. Under these conditions, the combustion always generates NO and NO.sub.2 which have destructive effects in nature, such as greenhouse effect, reduction of the stratospheric ozone layer, acid rains, etc.
Another method for the combustion of natural gas and other combustible gases is the catalytic oxidation. Classical catalytic combustors operate at temperatures of 600-900.degree. C. Methane, the main component of the natural gas, requires higher temperature for its combustion. The necessity to maintain a high temperature and the exothermic character of the reaction create problems when the process is carried out in a fixed bed reactor. The main problem is the formation of hot spots in the fixed bed, leading to thermal deactivation of the catalyst and the production of NO and NO.sub.2. The other disadvantage of classical catalytic combustors is their high sensitivity to the composition and temperature of the feed gas mixture, which creates a problem in controlling the process.
Another method for the combustion of hydrocarbons and combustible gases is the catalytic oxidation in a reactor with periodic flow reversal, conducted in a composite fixed bed containing a layer of heat exchange material on either side of the active catalyst bed. The gas flowing through the composite bed is controlled by a set of valves operating in tandem. The heat exchange layers of the bed function as preheaters for the feed gases or coolers for the combustion products. This function alternates depending on flow direction. Prior to the start up, the catalyst bed is preheated to the operating temperature. Unsteady-state operation is then provided by periodically reversing the direction of gas flow. Several modifications of this method have been proposed in U.S. Pat. Nos. 2,946,651, 3,870,474 and 5,364,259 as well as in Canadian Patent No. 1,154,933 for carrying out various processes. In U.S. Pat. No. 5,364,259, for example, volatile organic compounds found in gaseous process emissions are treated by catalytic oxidation in a regenerative incinerator. The disadvantage of the process described in the latter patent is the formation of hot spots that occurs in the reactor and leads to difficulties in controlling the temperature in the catalyst bed when the concentration of the volatile organic compounds is higher than 10 g/m.sup.3. This leads to a thermal destruction of the catalyst and production of NO.sub.x.